1. Field of the Invention
The present invention relates to amplifiers for condenser microphones, and in particular, to self-biased and phantom-powered amplifiers for electret microphones.
2. Description of the Related Art
Approximately 90 percent of all microphone applications presently use electret microphones. This type of condenser microphone has a permanently charged dielectric (electret) between two parallel metal plates (electrodes), one of which is attached to a diaphragm. This diaphragm moves in response to the pressure or particle velocity of sound waves, thereby changing the distance and, therefore, the capacitance, between the diaphragm and its electrode, or backplate. Since the amount of charge is fixed, the voltage between the diaphragm and backplate changes in a manner which is inversely proportional to the change in capacitance in accordance with Equation 1. EQU Q=C*V (1)
Referring to FIG. 1, a suitable model for an electret microphone is a capacitor Celectret connected in series with a voltage source Velectret. Because of the high output impedance and lack of current drive, the electret microphone requires buffering for its output signal. In most applications, for reasons of economy, the buffer amplifier is biased using phantom powering, i.e., the buffer amplifier receives its DC power and provides its AC output signal via a shared bias and signal path. This is accomplished by connecting the positive power supply line VDD to the buffer amplifier through a resistor Rload and decoupling the output signal through a coupling capacitor Cload.
The most commonly used device for the buffer amplifier is an N-channel depletion mode JFET (junction field effect transistor) with a reverse biased diode Dbias connected between the gate terminal and circuit ground GND. Because of the high input impedance of the gate terminal of the JFET, the leakage current of the diode Dbias will be sufficient to bias the gate terminal at ground potential GND. Meanwhile, the small signal impedance of the diode Dbias is sufficiently high so as to not affect the AC performance of the circuit.
While such use of a JFET is economical and allows for a compact circuit and since the gate terminal is always at ground potential, the JFET is always biased at the maximum drain current Idss and, therefore, does not require any start-up time to begin amplifying. However, disadvantages include low voltage gain, limited output voltage dynamic range, high total harmonic distortion and large variations in the drain current Idss from one device to another. For example, the voltage gain is typically less than unity which limits the output voltage range to that of the electret microphone (typically around 30 millivolts). This means that an external amplifier is required to increase the output voltage before conveying it to further signal processing circuits.
As for harmonic distortion, with the drain current Idss being a function of the square of the gate-to-source voltage Vgs of the JFET, non-linearity is high, typically around one percent at one kilohertz with 20 millivolts of output voltage. Additionally, variances in the drain current Idss from one device to another, and hence the gain of such devices, is around 50 percent, thereby requiring compensation by providing adjustable gain within the external amplifier.
Referring to FIG. 2, performance of the buffer amplifier can be improved by replacing the JFET with a more complex amplifier circuit. Feedback can be used to control the gain, improve the linearity and compensate for temperature variations. For example, if an operational amplifier is used, as shown, a DC bias loop with a low pass filter (LPF) is needed. Without such bias loop, the amplifier cannot operate as is shown. With the gate of P-type MOSFET (metal oxide semiconductor field effect transistor) M1 at ground potential due to the reverse biased nature of the diode Dbias, the loop gain, represented by Equation 2 (where G.sub.LOOP is the loop gain, G.sub.M1 is the gain of transistor M1, B is the operational amplifier gain and R.sub.LOAD is the load resistance), forces the potential of the gate terminal of P-MOSFET M2 to ground potential GND. This causes the circuit to not be biased properly and, therefore, unable to function as desired. EQU G.sub.Loop =G.sub.M1 * B* R.sub.LOAD (2)
Referring to FIG. 3, with a DC bias loop, however, the potentials of the gate terminals can be established at a higher voltage than ground GND and the amplifier can be properly biased. However, in order to get a good low frequency response, the components in the low pass filter (resistors R1 and R2 and capacitor C1) must be high in value and, therefore, relatively large in size and thus implemented as external components. Moreover, due to the large capacitance of the LPF capacitor C1, a long start-up time is required (e.g., approximately one second).